In general, in wireless communication, a radio wave transmitted from a transmitting station arrives at a receiving station through a plurality of transmission paths (multipath) and therefore a state occurs where the receiving station receives a plurality of path wavelets at various times. In this state, multipath fading occurs at the receiving station. Therefore, in a conventional wireless communication, a modulation/demodulation scheme with an anti-multipath property is used so as to prevent the multipath fading from deteriorating transmission characteristic.
As the modulation/demodulation scheme with an anti-multipath property, for example, a spread spectrum scheme, orthogonal frequency division multiplexing (OFDM) scheme, anti-multipath modulation scheme, and the like are used. The orthogonal frequency division multiplexing scheme is a scheme in which a packet is divided so as to be transmitted by using multiple subcarriers located over a wide band. The anti-multipath modulation scheme is a scheme in which anti-multipath property is represented by adding a phase redundancy and/or an amplitude redundancy in a transmission symbol. Even when wireless communication is performed by using a typical single carrier modulation scheme, the anti-multipath property is represented by using an equalizer in the receiving station.
Further, the spread spectrum scheme includes direct sequence spread spectrum (DSSS) scheme, frequency hopping spread spectrum (FHSS) scheme, time hopping spread spectrum (THSS) scheme, and the like. The direct sequence spread spectrum scheme is a scheme in which a spread code of a band wider than that of an unspread packet is multiplied by a packet so as to transmit the packet. The frequency hopping spread spectrum scheme is a scheme in which a transmission frequency for a packet is caused to hop over a wide band so as to transmit the packet. The time hopping spread spectrum scheme is a scheme in which a packet is spread by using an impulse signal of a wide band, and transmitted.
Further, the anti-multipath modulation scheme includes a PSK-VP (Phase Shift Keying with Varied Phase) scheme in which a convex phase redundancy is added (for example, Non-Patent Document 1), a PSK-RZ (Return to Zero Phase Shift Keying) scheme in which an amplitude redundancy is added (for example, Non-Patent Document 2), a DSK (Double Shift Keying) scheme (for example, Non-Patent Document 3), and the like. The single carrier modulation scheme includes, for example, a PSK (Phase Shift Keying) scheme, a QAM (Quadrature Amplitude Modulation) scheme, and the like.
Communication is performed by using such a modulation/demodulation scheme with an anti-multipath property so as to prevent deterioration of transmission characteristic caused by the multipath fading.
Further, in recent years, a study for a wireless transmission system (multihop system) for performing a wireless transmission by a plurality of wireless stations relaying packets from each other is proceeding. Conventionally, a technique using a modulation/demodulation scheme with an anti-multipath property has been suggested in this study (for example, Patent Document 1). Hereinafter, a conventional wireless transmission system 9 will be described with reference to FIG. 20. FIG. 20 is a diagram illustrating a configuration of the conventional wireless transmission system 9. In FIG. 20, the wireless transmission system 9 includes a transmitting station 91, relay stations 921 to 924, and a receiving station 93. The transmitting station 91 is a wireless station for initially transmitting a packet. The relay stations 921 to 924 are each a wireless station for relaying, to the receiving station 93, a packet transmitted by the transmitting station 91. The receiving station 93 is a wireless station which is a final destination to which the transmitting station 91 transmits a packet. Each wireless station uses the OFDM scheme as the modulation/demodulation scheme with an anti-multipath property. A packet is transmitted from the transmitting station 91 as indicated by dotted line arrows. The packet transmitted from the transmitting station 91 is received by each of the relay stations 921 to 924. The relay stations 921 to 924 simultaneously transmit the received packets, as indicated by solid linear rows. The packets transmitted by there lay stations 921 to 924 are received by the receiving station 93. Thus, the conventional wireless transmission system 9 causes the relay stations 921 to 924 to simultaneously transmit the packets, thereby reducing a time necessary for packet transmission. Each wireless station used for the conventional wireless transmission system 9 uses the OFDM scheme as the modulation/demodulation scheme with an anti-multipath property. Therefore, even when the packets which are simultaneously transmitted from the relay stations 921 to 924, respectively, arrive at the receiving station 93 at various arrival times, it is possible to prevent deterioration of transmission characteristic caused by the multipath fading.
Patent Document 1: Japanese Laid-Open Patent Publication No. 2000-115181
Non-Patent Document 1: H. Takai, “BER Performance of Anti-Multipath Modulation Scheme PSK-VP and its Optimum Phase-Waveform”, (IEEE, Trans. Veh. Technol.), Vol. VT-42, November, 1993, p 625-639
Non-Patent Document 2: S. Ariyavisitakul, S. Yoshida, F. Ikegami, K. Tanaka, T. Takeuchi, “A Power-efficient linear digital modulator and its application to an anti-multipath modulation PSK-RZ scheme”, (Proceedings of IEEE Vehicular Technology Conference 1987), June, 1987, p 66-71Non-Patent Document 3: S. Ariyavisitakul, S. Yoshida, F. Ikegami, T. Takeuchi, “A Novel Anti-Multipath Modulation Technique DSK”, (IEEE Trans. Communication), Vol. COM-35, No. 12, December, 1987, p 1252-1264